Offsetting Assets and Liabilities Cash and Money Market Investments (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets [Member] | Federal Funds Sold and Securities Borrowed or Purchased under Agreements to Resell [Member]
Gain (Loss) on Investments [Line Items]
Gross amounts recognized	$ 809	$ 568
Gross amounts offset in the balance sheet	(295)	0
Net amounts presented in the balance sheet	514	568
Gross amounts not offset in the balance sheet	0	0
Cash Collateral Received/Pledged	0	0
Net amount	514	568
Assets [Member] | Derivative Financial Instruments, Assets [Member]
Gain (Loss) on Investments [Line Items]
Gross amounts recognized	38	64
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	38	64
Gross amounts not offset in the balance sheet	(21)	(17)
Cash Collateral Received/Pledged	(1)	0
Net amount	16	47
Assets [Member] | Assets, Total [Member]
Gain (Loss) on Investments [Line Items]
Gross amounts recognized	847	632
Gross amounts offset in the balance sheet	(295)	0
Net amounts presented in the balance sheet	552	632
Gross amounts not offset in the balance sheet	(21)	(17)
Cash Collateral Received/Pledged	(1)	0
Net amount	530	615
Other Liabilities [Member] | Federal Funds Purchased and Securities Sold under Agreements to Repurchase [Member]
Gain (Loss) on Investments [Line Items]
Gross amounts recognized	5,271	827
Gross amounts offset in the balance sheet	(295)	0
Net amounts presented in the balance sheet	4,976	827
Gross amounts not offset in the balance sheet	0	0
Cash Collateral Received/Pledged	0	0
Net amount	4,976	827
Other Liabilities [Member] | Derivative Financial Instruments, Liabilities [Member]
Gain (Loss) on Investments [Line Items]
Gross amounts recognized	40	59
Gross amounts offset in the balance sheet	0	0
Net amounts presented in the balance sheet	40	59
Gross amounts not offset in the balance sheet	(21)	(17)
Cash Collateral Received/Pledged	(6)	(17)
Net amount	13	25
Other Liabilities [Member] | Liabilities, Total [Member]
Gain (Loss) on Investments [Line Items]
Gross amounts recognized	5,311	886
Gross amounts offset in the balance sheet	(295)	0
Net amounts presented in the balance sheet	5,016	886
Gross amounts not offset in the balance sheet	(21)	(17)
Cash Collateral Received/Pledged	(6)	(17)
Net amount	$ 4,989	$ 852